1. Field of the Invention
The present invention relates to magnetrons for producing high power electromagnetic radio frequency (RF) energy, and more particularly, to an output transition apparatus for a magnetron to couple the high power RF energy into an output waveguide.
2. Description of Related Art
Magnetrons have been used for many years in electronic systems that require high RF power in the microwave range, such as radar systems. A magnetron typically includes a cylindrical shaped cathode coaxially disposed within an anode structure, forming an interaction region between the cathode surface and the anode. The anode structure may include a network of vanes which provides a resonant cavity tuned to a frequency of oscillation for the magnetron. The RF microwave power produced by the magnetron is coupled into an output waveguide, which can direct the RF energy to a load, such as an antenna or other device.
Upon application of an electric potential between the cathode and the anode, which forms an electric field causing the cathode surface to emit a space-charge cloud of electrons. A magnetic field is provided along the cathode axis, perpendicular to the electric field, which causes the emitted electrons to spiral into cycloidal paths in orbit around the cathode. When RF fields are present on the anode structure, the rotating space-charge cloud is concentrated into a spoke-like pattern. This is due to the acceleration and retardation of electrons in regions away from the spokes. The electron bunching induces high RF voltages on the anode structure, and the RF levels build up until the magnetron is drawing full peak current for any given operating voltage. Electron current flows through the spokes from the cathode to the anode, producing a high power RF output signal at the desired frequency of oscillation.
Since desirable operation of the magnetron is achieved by maintaining a vacuum environment within the magnetron, and there is a non-vacuum environment within the output waveguide, a vacuum barrier must be provided between the magnetron and output waveguide. One type of magnetron utilizes a glass dome as a vacuum barrier between the vacuum and non-vacuum environments. The electromagnetic signal is coupled into the space formed below the dome, and the signal radiates through the dome. The glass material of the dome provides an RF transparent barrier which allows the signal to be effectively transported into the output waveguide, while maintaining a vacuum seal within the magnetron.
One drawback with this approach is that the glass material of the dome cannot withstand high temperatures. It is common to perform a high temperature processing of the magnetron components, including the cathode and anode structure, during fabrication processing of the magnetron. This high temperature processing tests the temperature limits of the device, and forces certain chemical elements of the components into a vaporous state at which they can be removed from the device. Without performing this high temperature processing, these elements could be emitted during operation of the magnetron. Emission of the chemical elements could alter the electrical and resonant characteristics of the magnetron in an undesirable manner. Thus, it is preferred to perform the high temperature processing at as high a temperature as the magnetron can withstand.
However, the glass dome which provides the vacuum barrier between the magnetron and the output waveguide is generally unable to withstand such high temperatures. The glass material would tend to soften, resulting in reduction of structural integrity of the dome, and possible loss of the vacuum environment. To remedy this situation, the high temperature processing must be conducted at a temperature level somewhat below the critical temperature for the glass dome. This reduces the effectiveness of the high temperature processing, and limits the operational range of the magnetron.
As an alternative to reducing the effectiveness of the high temperature processing, other materials have been suggested for formation of the dome, such as ceramic. Ceramic materials are known to be capable of withstanding very high temperatures and could be a suitable material to replace the glass dome. However, ceramic is relatively difficult and expensive to form into a dome shape conducive to effective transmission of RF energy using conventional machining techniques.
Accordingly, a need exists to provide a magnetron transition apparatus which provides for efficient coupling of electromagnetic RF energy between the vacuum environment of the magnetron and the non-vacuum environment of the output waveguide. The magnetron transition apparatus should be capable of withstanding high temperature processing during manufacture of the magnetron components, and should be relatively inexpensive to fabricate.